The present invention relates to load sensing fluid controllers, and more particularly, to such controllers in which the load signal level is capable of changing, depending upon the deflection of the controller valving.
It will become apparent to those skilled in the art that the present invention may be used advantageously with any type of fluid controller which is operable to control the flow of fluid from a source of pressurized fluid to a fluid operated device, wherein the source of fluid includes pressure responsive means for varying the delivery of fluid to the controller. However, the present invention is especially useful when applied to controllers such as the steering control unit of a full fluid-linked vehicle steering control system, and the invention will be described in connection therewith.
Several trends in the development of hydraulic steering control units (SCU's) are relevant to the present invention. First, as in hydraulics generally, the use of load sensing is becoming increasingly important for reasons which are now well known to those skilled in the art, such as minimizing the amount of energy consumed by the source of pressurized fluid (the pump). Second, in regard to full fluid-linked steering specifically, SCU's of the type described herein are being used on increasingly larger vehicles, thus requiring greater flow capacities.
A typical SCU, of the type to which the present invention relates, includes valving, a fluid meter, and an arrangement for imparting follow-up movement to the valving in synchronism with the flow through the fluid meter. The flow through the SCU is directly proportional to the flow area of the main flow control orifice (which is proportional to the rate at which the steering wheel is rotated), and is also proportional to the square root of the pressure drop across the main flow control orifice. As a result, increasing the flow capacity of an SCU has necessitated increasing the flow area of the various valving passages and orifices and, as the result, increasing the overall size of the SCU.
Attempts have also been made to increase the flow capacity of the SCU by increasing the pressure drop across the main flow control orifice. This provides a satisfactory flow rate at the higher valve deflections, but results in an excessive flow and gain rate through the valving for lower valve deflections. Increasing the pressure drop across the main flow control orifice also increases the standby pressure delivered to the SCU, increasing the potential for leakage through a closed center SCU (causing vehicle "drift"), and wasting pump energy.
It is known in the load sensing directional spool valve art to overcome the latter problem by communicating the load signal with tank when the valve is in neutral and bleeding signal fluid through a synthetic signal generator and into the main flow path, downstream of the main flow control orifice, when the valve is shifted from neutral. See U.S. Pat. Nos. 3,815,477 and 3,971,216. Thus, the prior art provides a relatively lower load signal when the valve is in neutral, and a relatively higher load signal when the valve is away from neutral, anywhere in its operating range.
Another problem associated with fluid controllers of the type to which this invention relates is steering wheel "precession", i.e., the position of the steering wheel corresponding to the neutral position of the controller "precesses" or moves slowly in one direction or the other during operation of the system. Another problem relates to internal leakage, within the valving, between "metered" and "unmetered" fluids. These and several other related problems have been largely overcome by the "symmetrical" valving arrangements for fluid controllers disclosed and claimed in U.S. Pat. Nos. 4,033,377 and 4,109,679, assigned to the assignee of the present invention. The fluid controller of the present invention utilizes the valve symmetry disclosed in the above-cited patents.